Preheat oven to 450 degrees F. Line a 15x10x1-inch baking pan with foil; set aside. With a sharp knife or vegetable peeler, cut zucchini lengthwise in thin slices (you'll need 8 slices). Cut pork tenderloin crosswise into 4 equal portions. Press meat down with the palm of hand to flatten slightly.
Wrap each tenderloin portion with two zucchini slices (reserve remaining zucchini for another use.) Place in prepared pan. Lightly brush with oil; sprinkle with salt and pepper.
Roast, uncovered, 18 to 20 minutes (12 ounces) or 25 to 30 minutes (16 ounces), or until meat registers 160 degrees F. Spoon some of the pesto over each tenderloin just before serving and sprinkle with basil leaves. Serve with remaining pesto and watercress. Makes 4 servings.